Bluetooth: RFCOMM: fix NULL dereference of dlc->session in RFCOMM_CONNINFO

In the Linux kernel, the following vulnerability has been resolved:

Bluetooth: RFCOMM: fix NULL dereference of dlc->session in RFCOMM_CONNINFO

The RFCOMM_CONNINFO getsockopt handler accepts a socket that is not
connected as long as deferred setup is enabled:

if (sk->sk_state != BT_CONNECTED &&
!rfcomm_pi(sk)->dlc->defer_setup) {
err = -ENOTCONN;
break;
}
l2cap_sk = rfcomm_pi(sk)->dlc->session->sock->sk;

dlc->defer_setup is set in rfcomm_sock_init() when rfcomm_connect_ind()
creates a child socket for an incoming connection on a listening socket
that has BT_DEFER_SETUP enabled. It is never cleared afterwards. The
session, however, can go away underneath it.

rfcomm_recv_disc() forces the dlc state before tearing it down:

d->state = BT_CLOSED;
__rfcomm_dlc_close(d, err);

The RFCOMM_DEFER_SETUP early return in __rfcomm_dlc_close() only covers
BT_CONNECT, BT_CONFIG, BT_OPEN and BT_CONNECT2, so with the state
already BT_CLOSED that switch does not match and the function falls
through to rfcomm_dlc_unlink(), which sets d->session = NULL, while
d->defer_setup stays 1.

A getsockopt(SOL_RFCOMM, RFCOMM_CONNINFO) on the accepted socket after
that point therefore skips the -ENOTCONN path -- sk->sk_state is
BT_CLOSED, but dlc->defer_setup is still set -- and dereferences the
NULL session. No race is needed: once the DISC has been processed, the
dereference is unconditional.

Reproduced on a KASAN kernel under QEMU with a BR/EDR peer emulated over
/dev/vhci: the peer brings up an ACL link, opens L2CAP on the RFCOMM
PSM, starts a session and sends SABM for a channel bound with
BT_DEFER_SETUP, and sends DISC for that dlci after the socket has been
accepted. getsockopt(SOL_RFCOMM, RFCOMM_CONNINFO) on the accepted
socket then hits:

Oops: general protection fault, probably for non-canonical address
0xdffffc0000000002: 0000 [#1] SMP KASAN PTI
KASAN: null-ptr-deref in range [0x0000000000000010-0x0000000000000017]
CPU: 1 UID: 0 PID: 150 Comm: init Tainted: G B 7.3.0-rc3-g5dd1818b15d9
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996)
RIP: 0010:rfcomm_sock_getsockopt+0x529/0x780
Call Trace:
<TASK>
do_sock_getsockopt+0x3ad/0x7d0
__sys_getsockopt+0x10e/0x1b0
__x64_sys_getsockopt+0xc2/0x160
do_syscall_64+0xda/0x4b0
entry_SYSCALL_64_after_hwframe+0x77/0x7f
</TASK>

0x10 is the offset of sock in struct rfcomm_session;
rfcomm_sock_getsockopt_old() is inlined into rfcomm_sock_getsockopt().

Commit 43a556b2fd43 ("Bluetooth: RFCOMM: take rfcomm_mutex for the
deferred setup accept") fixed the same "a remote DISC clears the session
while deferred setup is still flagged" problem in rfcomm_dlc_accept();
this is the remaining instance of it, in the getsockopt path.

Deferred setup only leaves a socket usable here once it has reached
BT_CONNECT2, so restrict the exception to that state and check that a
session is actually present before following it.